This invention relates generally to the field of syringes for injecting a medicinal liquid into a patient (human or animal) via a hypodermic needle. In particular, it relates to a syringe adapted for use with a disposable needle and a self-contained, disposable reservoir or ampoule of medicinal liquid, wherein the liquid is discharged from the ampoule by compressing or squeezing the ampoule.
Conventional hypodermic syringes comprise a tubular reservoir with a fitting at one end for the attachment of a hypodermic needle. The other end of the reservoir is provided with a plunger. Syringes of this type must be filled from a separate vial or ampoule in the well known manner of pushing the plunger all the way into the reservoir, inserting the needle into the vial, and pulling the plunger out until the reservoir is filled to the desired level, as indicted by volumetric markings on the reservoir wall. Before injection, a small amount of liquid must be expressed through the needle to assure that no air is injected. This is especially important with intravenous injections, where injected air could cause a fatal embolism.
Several disadvantages of these conventional syringes have been recognized. For example, the need to fill the syringe from a separate vial is cumbersome and time-consuming, as is the need precisely to measure the dosage and then express the liquid to remove air from the syringe. Also, the plunger mechanism can be awkward to manipulate in certain situations, and, in any case, requires at least some degree of dexterity. This can present a problem in certain applications, such as in the application of local anesthesia, where an injection must be made into a relatively inaccessible part of the body. Patients, such as diabetics, who must perform self-injections, and particularly those who are elderly, weak, or infirm, sometimes find it difficult to inject themselves with the plunger-type syringe.
Accordingly, attempts have been made to provide alternative syringe designs which overcome the disadvantages of the plunger mechanism. One approach has been to provide a syringe with a self-contained reservoir or ampoule pre-filled with a precisely measured amount of medicament. The ampoule has flexible walls, so that its contents can be discharged by compressing or squeezing it. Devices of this type are exemplified in the following U.S. patents:
______________________________________ U.S. Pat. No. Patentee ______________________________________ 798,093 Dean 2,618,263 Lakso, et al. 3,099,264 Hubbard 3,114,369 Hall 4,013,073 Cunningham 4,475,906 Holzner ______________________________________
While the above-listed prior art devices do, in varying degrees, avoid the aforementioned problems associated with the plunger-type syringe, none has found widespread acceptance by medical practitioners, for a variety of reasons. For example, some of the prior art devices offer insufficient physical support for the needle during injection, making their use awkward at best. Others require a degree of dexterity in their use that, while possibly an improvement over the plunger-type syringe, still renders them ill-suited for self-injection, especially by the physically feeble. Another drawback in some prior art devices is their inability to be used with disposable needles. In such devices, the syringe and needle are manufactured as a disposable unit, a practice some may consider wasteful. Also, in devices of this type, some mechanism must be included to prevent the flow of liquid from the reservoir into the needle until an injection is to be performed. Such a mechanism adds to the complexity, and thus the cost, of such devices.
There has thus been a long-felt, but not entirely satisfied, need for a syringe that contains a pre-measured reservoir or ampoule of medicament, which provides good physical support for the needle and which offers an injection mechanism that is easy to manipulate without much dexterity or strength. The need has also been felt for a syringe of this nature which can use disposable needles, and which, therefore, does not require means for blocking the flow from the reservoir into a pre-attached needle.